High-performance fiber-polymer composites are rapidly achieving a prominent role in the design and construction of military and commercial aircraft, sports and industrial equipment, and automotive components. Composites fill the need for stiffness, strength, and low weight that cannot be met by other materials. The most widely utilized high-performance fiber-polymer composites are composed of oriented carbon (graphite) fibers embedded in a suitable polymer matrix. To contribute high strength and stiffness to the composite, the fibers must have a high aspect ratio (length to width). Fibers may be chopped or continuous. The mechanical properties of chopped fiber composites improve greatly as the aspect ratio increases from 1 to about 100. Mechanical properties still improve but at a slower rate for further increases in aspect ratio. Therefore, aspect ratios of at least about 25, and preferably of at least about 100 are desirable for chopped fiber composites. Composites prepared with continuous fibers have the highest stiffness and strength. Fabricating fiber-containing composites, however, requires significant manual labor and such composites cannot be recycled. Furthermore, defective and/or damaged composite materials cannot be easily repaired.
Molecular composites are high-performance materials which are much more economical and easier to process than conventional fiber-polymer composites. In addition, molecular composites can be recycled and are repairable. Molecular composites are composed of polymeric materials only, i.e., they contain no fibers. Such molecular composites can be fabricated much more simply than fiber-polymer composites.
Molecular composites are materials composed of a rigid-rod polymer embedded in a flexible polymer matrix. The rigid-rod polymer can be thought of as the microscopic equivalent of the fiber in a fiber-polymer composite. Molecular composites with the optimum mechanical properties will contain a large fraction, at least 30 percent, of rigid-rod polymers, with the balance being polymeric binder. Molecular composites may contain either oriented or unoriented rigid-rod polymers.
A molecular composite requires that the rigid-rod polymer be effectively embedded in a flexible, possibly coil-like, matrix resin polymer. The flexible polymer serves to disperse the rigid-rod polymer, preventing bundling of the rigid-rod molecules. As in conventional fiber/resin composites, the flexible polymer in a molecular composite helps to distribute stress along the rigid-rod molecules via elastic deformation of the flexible polymer. Thus, the second, or matrix-resin, polymer must be sufficiently flexible to effectively surround the rigid-rod molecules while still being able to stretch upon stress. The flexible and rigid-rod polymers can also interact strongly via Van der Waals, hydrogen bonding, or ionic interactions. The advantages of molecular composites can only be realized with the use of rigid-rod polymers.
Most of the linear polymers produced commercially today are coil-like polymers. The chemical structure of the polymer chain allows conformational and rotational motion along the chain so that the entire chain can flex and adopt coil-like structures. This microscopic property relates directly to the macroscopic properties of flexural strength, flexural moduli, and stiffness. If fewer or less extensive conformational changes are possible, a stiffer polymer will result.
Two technical difficulties have limited molecular composites to laboratory curiosities. Firstly, the prior art relating to molecular composites calls for merely blending or mixing a rigid-rod polymer with a flexible polymer. It is well known in the art, however, that, in general, polymers of differing types do not mix. That is, homogeneous single phase blends cannot be obtained. This rule also applies to rigid-rod polymers and, thus, the early molecular composites could be made with only small weight fractions of a rigid-rod component. In these systems, an increase in the fraction of the rigid-rod component led to phase separation, at which point a molecular composite could no longer be obtained.
The second technical difficulty is that rigid-rod polymers of significant molecular weight are exceedingly difficult to prepare. The technical problem is exemplified by the rigid-rod polymer, polyparaphenylene. During the polymerization of benzene, or other monomer leading to polyparaphenylene, the growing polymer chain becomes decreasingly soluble and precipitates from solution causing the polymerization to cease. This occurs after the chain has grown to a length of only six to ten monomer units. These oligomers, i.e., rigid-rod polymers, are too short to contribute to the strength of a composite. A lack of solubility is a general property of rigid-rod polymers, hence, synthesis of all such rigid-rod polymers is difficult.
The solubility problem may be avoided in the special case in which the product polymer contains basic groups which can be protonated in strong acid and the polymerization can be conducted in strong acid. For example, rigid-rod polyquinoline can be prepared in the acidic solvent system dicresylhydrogenphosphate/m-cresol, because the quinoline group interacts with the acidic solvent, preventing precipitation. However, the resulting polymers are soluble only in strong acids, making further processing difficult.
Before molecular composites can become a practical reality, the problems of (a) blending the rigid-rod and flexible components into a stable homogeneous phase, and (b) the low solubility of the polymer, must be overcome.